The present invention relates to voltage controlled oscillators (VCOs) and, more particularly, to a circuit for bandswitching a VCO while keeping control line sensitivity substantially constant.
Voltage controlled oscillators having an output frequency variable over some predetermined range according to an input control voltage are well known elements of communication equipment such as radios and televisions. It is also well known that in bandswitching a VCO using fixed capacitors (i.e. moving from a first to a second predetermined frequency band), the sensitivity of the VCO tank circuit changes, the sensitivity being related to the variation in output frequency with changing input control voltage. Generally, and absent some circuity to compensate for the effect, the frequency variation in the high band is different than (larger or smaller) the frequency variation in the low band, given the same change in input control voltage and depending on where the capacitance is added in the resonant circuit.
To compensate for this effect of varying sensitivity, the prior art includes circuits in which inductors are switched to maintain constant sensitivities when changing frequency ranges, or complex VCO designs utilizing multiple varactors or switches are employed. Examples of the foregoing are U.S. Pat. No. 3,813,615 to Okazaki, U.S. Pat. No. 3,825,858 to Amemiya et al, and U.S. Pat. No. 3,866,138 to Putzer.
While the foregoing patents primarily involve switching inductors, there are a number of reasons why switching capacitors is more desirable. One reason is that tapped or multiple coils are difficult to finely proportion for accurate frequency switching. Maintaining good tank quality factor, Q, related to the sharpness of the resonance frequency peak, can also be more difficult with inductor switching; and multiple coils take needed space in this era of miniaturization of electronic circuitry. However, simple capacitor changes either need more complex VCO designs or they change VCO sensitivity drastically. Accordingly, complex VCO designs have been employed, but they require multiple electronic switches which, in turn, utilize multiple components which can be expensive, space consuming and/or unreliable.